National treasures ... water lilies in the Pantanal Matogrossense National Park in Brazil. Photograph: Theo Allofs/Corbis

A reasonable maxim to apply to bus travel in Brazil is that the older and more decrepit the bus, the worse the roads on its route. As less than 20% of the country's roads are asphalted, there's a good chance of driving over mud and gravel almost everywhere.

So when the bus was ready to leave Sento-Sé in Bahia, its battered state was no surprise. Our inquiry about departures had been met with a shrug and a list of possible times. We accepted the advice to come back the following day when more might be known.

The forecast journey time was five hours. For the first four we jolted across mud ruts. The principal entertainment was watching the driver weave between herds of goats and cattle that believed they had exclusive ownership of the road. We were spared air conditioning, which is no bad thing as it is usually set so low that passengers require winter woollies and a blanket even when the temperature outside is in the high 20s or 30s.

We were coming back from the Parque Nacional Serra da Capivara in the north-eastern state of Piauí. There aren't any capivaras in the 1,300 sq km of Serra da Capivara. If the world's biggest rodents – weighing up to 80kg – ever lived there, they left a long time ago. The park does, however, have more than 100 sites containing prehistoric rock paintings. Their discovery in the 1970s revealed that Homo sapiens lived in South America at least 100,000 years ago, much earlier than was previously believed. That is its prime interest.

Little information is available through websites on travel in the interior. It's a matter of asking around, which is how we came to cross Velho Chico (the São Francisco), one of Brazil's most famous rivers. The ferry, we discovered, might sail the following Friday. But, as it was Sunday, it was possible that a boat owner who took passengers would be returning after visiting his family for the weekend. Fortunately he did, though the boat berthed on the edge of a marsh almost a kilometre from the town. Three buses and 29 hours later we arrived home.

All this explains why, in spite of its attractions, Brazil's Parque Nacional Serra da Capivara gets only 1,000 or so visitors a year while its more accessible neighbours receive more than 50 times that number.